Method and apparatus for determining characteristics of a laser beam spot

ABSTRACT

A method of determining the dimensions of a laser beam spot, comprising: scanning the laser beam in a path across a reference-edge having a photodetector positioned therebehind; and measuring an output signal from the photodetector during the scanning, the output signal corresponding to an area of the laser beam spot incident on the photodetector during the scanning. 
     A method of aligning a laser beam delivery system, the method comprising: positioning a measurement/alignment tool at a target location; firing the laser beam on the tool; observing the laser beam using the tool; and adjusting the system in response to the sensed laser beam.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit of priorityfrom U.S. patent application Ser. No. 09/395,809, filed Sep. 14, 1999,now U.S. Pat. No. 6,559,934, the full disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to calibration techniques for determiningthe characteristics of a laser beam, particularly for use with laser eyesurgery systems. More specifically, the invention provides devices,systems, and methods for determining the dimensions and/or position ofthe laser beam spot upon a target, and can provide input for generating,verifying, or adjusting ablation algorithms used to plan a resculptingprocedure. When used in conjunction with laser eye surgery systems, thepresent invention can assist in determining patterns of laser beam spotdelivery upon a patient's cornea, and can also be used in calibratingthe laser beam delivery system.

BACKGROUND OF THE INVENTION

When performing laser eye surgery such as when ablating a target regionon a patient's cornea with a refractive laser beam system, it isbeneficial to have accurate information on the dimensions of the laserbeam spot which is incident on the cornea. Deviation from a desired spotsize and shape, such as by increased or decreased diameter of the laserbeam spot or by the spot exhibiting an oval or non-symmetrical shape,could result in tissue ablation at undesired locations on the patient'scorneas with each laser pulse, leading to less than ideal resculpting.Inaccuracy in the location of the laser spots may result in off-centerablations.

SUMMARY OF THE INVENTION

The present invention provides methods and apparati for determiningcharacteristics of a laser beam spot, the characteristics typicallyincluding the intensity, dimensions, and/or position of the laser beamspot. An advantage of the present invention is that it can be used withlaser eye surgery systems such that the dimensions of the laser beamspot, (including its diameter, area and eccentricity), can be preciselydetermined prior to, or concurrently with, the laser beam spot beingused to ablate a region of the patient's cornea.

In preferred methods of the present invention, a laser beam is scannedin a path across a reference-edge, (which may preferably comprise aknife-edge), having a photodetector positioned therebehind, with thelaser beam preferably remaining in a path generally perpendicular to theplane of the reference-edge during the scanning.

An output signal is generated by the photodetector corresponding to apercentage of the laser beam which is actually incident on thephotodetector, (ie: not blocked by the reference-edge), at variousmoments in time during the scanning of the laser beam. For a beam havinga uniform energy distribution, the percentage of the laser beam energywhich is incident on the photodetector will correspond to the area ofthe laser beam spot which is incident on the photodetector. By measuringthe output signal characteristics of the photodetector during thescanning, the present invention provides systems for determining thesize and shape of the laser beam spot as well as the intensity of thelaser beam. In preferred aspects, a computer calculates the intensityand shape profiles of the laser beam from the photodetector outputsignals.

As stated, the output signal generated by the photodetector willcorrespond to the size of the area of the laser beam spot incidentthereon. As such, when the laser beam is fully incident on thereference-edge, (ie: when it is blocked from reaching the photodetectorby the reference-edge), the photodetector will generate no outputsignal, or it will only generate a minimal output signal as a result ofnoise. Conversely, when the laser beam spot has been scanned completelyacross the reference-edge and is then fully incident on thephotodetector, the photodetector will generate a maximum output signal.

The larger the area of the laser beam spot incident upon thephotodetector, the stronger the output signal generated by thephotodetector. Accordingly, in a preferred aspect of the invention, theintensity of the laser beam is determined by measuring the maximumoutput signal of the photodetector when the laser beam spot is fullyincident on the photodetector and is not blocked by the reference-edge.

In another preferred aspect of the invention, the total area of thelaser beam spot is determined by integrating the area under a curverepresenting the intensity of the photodetector signal output during thescanning as the laser beam is scanned across the reference-edge.

In yet another preferred aspect of the invention, the position of thecenter of the laser beam spot is located by determining when the outputsignal of the photodetector reaches half of its maximum output signalduring the scanning, thus indicating that the center of the laser beamspot is positioned directly at the edge of the reference-edge, (with onehalf of the laser beam spot incident on the photodetector and one Thelarger the area of the laser beam spot incident upon the photodetector,the stronger the output signal generated by the photodetector.Accordingly, in a preferred aspect of the invention, the intensity ofthe laser beam is determined by measuring the maximum output signal ofthe photodetector when the laser beam spot is fully incident on thephotodetector and is not blocked by the reference-edge.

In another preferred aspect of the invention, the total area of thelaser beam spot is determined by integrating the area under a curverepresenting the intensity of the photodetector signal output during thescanning as the laser beam is scanned across the reference-edge.

In yet another preferred aspect of the invention, the position of thecenter of the laser beam spot is located by determining when the outputsignal of the photodetector reaches half of its maximum output signalduring the scanning, thus indicating that the center of the laser beamspot is positioned directly at the edge of the reference-edge, (with onehalf of the laser beam spot incident on the photodetector and one halfof the laser beam spot incident on the reference-edge).

In another preferred aspect of the present invention, the width of thelaser beam spot in the direction of the path of the scanning isdetermined by locating the positions of the leading and trailing edgesof the laser beam spot and then determining a spacing therebetween. Inthis aspect of the invention, the leading edge of the laser beam spot islocated by determining when the photodetector begins to emit an outputsignal, (being indicative of the laser beam spot leading edge firstpassing over the reference-edge and becoming incident on thephotodetector). The trailing edge of the laser beam spot is located bydetermining when the output signal of the photodetector has reached amaximum (indicating that the laser beam spot is not blocked by thereference-edge and is therefore fully incident on the photodetector).After determining the moments in time when the leading and trailingedges of the laser beam spot pass over the reference-edge as set outabove, the width of the laser beam spot in the direction of the scanningis calculated based upon the speed of the laser beam scanning across thereference-edge.

In another preferred aspect of the present invention, the width of thelaser beam spot in the direction of the path of the scanning isdetermined by locating the positions of the leading and trailing edgesof the laser beam spot and then determining a spacing therebetween. Inthis aspect of the invention, the leading edge of the laser beam spot islocated by determining when the photodetector begins to emit an outputsignal, (being indicative of the laser beam spot leading edge firstpassing over the reference-edge and becoming incident on thephotodetector)., The trailing edge of the laser beam spot is located bydetermining when the output signal of the photodetector has reached amaximum (indicating that the laser beam spot is not blocked by thereference-edge and is therefore fully incident on the photodetector).After determining the moments in time when the leading and trailingedges of the laser beam spot pass over the reference-edge as set outabove, the width of the laser beam spot in the direction of the scanningis calculated based upon the speed of the laser beam scanning across thereference-edge.

In other aspects of the present invention, asymmetries andeccentricities in the laser beam spot are found by measuring the rate ofchange or the symmetry of the rate of change of the output signal duringthe scanning.

In yet other aspects of the present invention, the size, shape andposition of the laser beam spot are determined in two directions whichare preferably perpendicular to one another. In this aspect of theinvention, scanning is preferably performed in two perpendicular paths,over perpendicular first and second reference-edges. In this aspect ofthe invention, the size, shape and position of the laser beam spot aredetermined in the two perpendicular directions by measuring the outputsignals from either a single photodetector or two separatephotodetectors positioned behind the reference-edges. An advantage ofthis aspect of the invention is that asymmetries of the beam spot (ie:an irregular shape of the beam spot) as well as eccentricities of thebeam spot (ie: elongation of the beam spot to form an oval-shape), canbe detected.

In preferred aspects of the present invention, the photodetector is abulk detector. As such, an advantage of the present invention is that amore complex and expensive imaging detector is not required.

The present invention also provides methods of calibrating scanninglaser beam delivery system. These methods comprise positioning acalibration tool at a target location; directing the laser beam onto thetool; sensing the laser beam using the tool; and adjusting the system inresponse to the sensed laser beam. In various aspects, the laser beamcan be repeatedly re-directed, (for example, by a galvanometric mirror),between the tool and a patient's cornea. As such, after determining thesize, shape and/or position of the beam, the laser beam can be appliedat a known location on the cornea. Alternatively, the tool can berepeatedly inserted into and removed from the beam path between thelaser beam source and the patient's cornea. As such, the alignment toolcan then be repeatedly removed from the target location to allow forresculpting of the patient's cornea and then replaced at the targetlocation after the resculpting of the cornea. Using either approach, arepetitive measurement of intensity and shape characteristics of thelaser beam can be made as well as repetitive recallibration of thetargeting of the laser beam can be achieved, thus ensuring precisepositional accuracy when ablating the patient's cornea.

In still further aspects of the invention, the laser beam is split witha first portion of the beam directed to the measurement/alignment tooland a second portion directed to the patient's cornea such that realtime measurement of shape and intensity characteristics of the laserbeam spot and/or real time alignment of the laser beam delivery systemcan be achieved.

Regardless of the tool positioning, the calibration tool will oftenprovide signals indicating beam spot size, shape, energy distribution,and/or location. These signals may be used to adjust the plannedablation protocol of the beam delivery system. Specifically, using thesensed information, an algorithm for calculating the locations andnumber of shots can be revised, thereby increasing the accuracy of theresculpting procedure. This calibration information can be used toadjust the ablation algorithm immediately before and/or during eachablation procedure.

In other aspects of the present invention, the measuring/alignment toolcomprises a target which fluoresces in response to laser light incidentthereon. In this second embodiment of the invention, an operator viewsthe position of the fluoresced spot on the target screen while directinglaser light at the target screen. Such viewing may preferably be donethrough the system microscope. The beam delivery system is aligned withthe targeting optics, which may comprise a cross-hair reticle, therebycalibrating the laser beam delivery system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser beam being scanned over areference-edge having a photodetector positioned therebehind at themoment in time when the laser beam is centered over the reference-edge.

FIG. 2 is a top plan view corresponding to FIG. 1.

FIGS. 3A, 3B and 3C are sequential illustrations of the laser beammoving across the reference-edge of FIGS. 1 and 2.

FIG. 4 is a graph of the output signal of the photodetector during thescanning illustrated in FIGS. 3A, 3B and 3C.

FIG. 5 is a view of an oval shaped laser beam spot, (having a major axisparallel to the path of the scanning), being scanned over areference-edge with a photodetector positioned therebehind.

FIG. 6 is a representation of the output signal of the photodetectorduring a scanning of the oval shaped laser beam spot of FIG. 5.

FIG. 7 is a plan view of an oval shaped laser beam spot, (having a majoraxis perpendicular to the path of the scanning), being scanned over areference-edge with a photodetector therebehind.

FIG. 8 is a representation of the output signal of the photodetectorduring a scanning of the oval shaped laser beam spot of FIG. 7.

FIG. 9 is a plan view of an eccentric shaped laser beam spot beingscanned over a reference-edge with a photodetector therebehind.

FIG. 10 is a representation of the output signal of the photodetectorduring a scanning of the oval shaped laser beam spot of FIG. 9.

FIG. 11 is a top plan view of a laser beam spot being scanned over twoperpendicular reference-edges wherein the two reference-edges togetherform a corner of a planar member.

FIG. 12 corresponds to FIG. 11, but instead uses two separatedphotodetectors.

FIG. 13 is a top plan view showing a laser beam scanning over twoperpendicular reference-edges, each reference-edge having a separatephotodetector positioned therebehind.

FIG. 14 is a perspective view of the laser beam delivery systemdirecting a laser beam at a screen which fluoresces in the region wherethe laser beam is incident thereon.

FIG. 15A is a view through the targeting optics of the laser beamdelivery system prior to system calibration when the laser beam isdirected to the fluorescing screen of FIG. 14.

FIG. 15B is a view corresponding to FIG. 15A, after system calibration.

FIG. 16 is an illustration of the laser beam delivery system scanning alaser beam across a calibration tool and applying a therapeutic laserbeam to a patient's cornea.

FIG. 17 is an illustration of the laser beam delivery system applying atherapeutic laser beam to a patient's cornea showing a removablecalibration tool in the beam path.

FIG. 18 is an illustration of the laser beam delivery systemsimultaneously applying a therapeutic laser beam to a patient's corneaand to a calibration tool.

FIG. 19 is an illustration of the laser beam delivery system directing alaser beam through selectable apertures of an aperture wheel or turret.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

FIGS. 1 to 13 illustrate various aspects of a first embodiment of thepresent invention. FIGS. 14 to 15B illustrate various aspects of asecond embodiment of the present invention. FIGS. 16 to 20 illustratecalibration systems which include a calibration tool which may comprisethe first or second embodiment of the present invention.

When targeting an excimer laser beam to ablate regions of a patient'scornea during laser eye surgery, the spot formed by the laser beam uponthe target will often have a circular shape, and will typically beintended to have a substantially uniform energy distribution. Otherknown beam delivery systems have rectangular or slit-shaped beams,optionally with Gaussian or other uneven energy profiles. Regardless,the exact intensity and shape profiles of the laser beam spot can notalways be determined relying upon th targeting optics of the laserdelivery system alone. It is beneficial to know the intensity and shapeprofiles of the laser beam as accurately as possible, especially whengenerating a pattern of laser beam spot application to the patient'scornea. Having accurate intensity and shape profile for the laser beamspot, it is possible to accurately sculpt the patient's cornea throughsuccessive application of a laser beam in a pattern of spots on thecornea. The present invention provides accurate determination ofintensity and shape profiles of the laser beam spot which can be used togenerate targeting patterns, and to otherwise calibrate the system.

The laser system may include, but is not limited to, an excimer lasersuch as an argon-fluoride excimer laser producing laser energy with awavelength of about 193 nm. Alternative laser systems may include solidstate lasers, such as frequency multiplied solid state lasers,flash-lamp and diode pumped solid state lasers, and the like. Exemplarysolid state lasers include UV solid state lasers producing wavelengthsof approximately 193–215 nm such as those disclosed in U.S. Pat. Nos.5,144,630, and 5,742,626, and in Borsuztky et al., “Tunable UV Radiationat Short Wavelengths (188–240 nm) Generated by Frequency Mixing inLithium Borate,” Appl. Phys. 61:529–532 (1995). A variety of alternativelasers might also be used. The laser energy will often comprise a beamformed as a series of discreet laser pulses or shots.

The exact diameter and shape of the laser beam spot upon a target cannot always be precisely determined relying upon the targeting systemoptics alone. This is especially true if the shape of the laser beamspot is somewhat eccentric or asymmetrical. Moreover, minor changes inthe size and shape of the laser beam spot can be introduced whenswitching between different apertures and lenses in the laser deliverysystem. For example, FIG. 19 illustrates a laser beam 18 passing throughan aperture 210 of an aperture wheel 200. As wheel 200 is rotated, laserbeam 18 will pass through various apertures 220, 230 and 240. Each ofapertures 210, 220, 230 and 240 may preferably be sized to differentdiameters such that different diameters of beam 18 can be selectivelyapplied to the patient's cornea. The present invention provides systemswhich can determine the precise size and shape of beam 18 as it passesthrough each of apertures 210, 220, 230 and 240, as explained herein.

The present invention provides methods and apparati for preciselydetermining dimensions including the size, shape and position of thelaser beam spot upon the target. Accordingly, laser beam spot shape andintensity profiles can be generated for use in sculpting the patient'scornea with a pattern of laser beam spots thereon. Also, the targetingoptics of the laser delivery system can be aligned to account for anyoffset between the actual position of the laser beam as determined bythe present invention and the position of the laser beam as determinedby the scanning hardware and galvanometers of the laser deliverysystem's targeting optics. By determining the exact size, shape andintensity of the laser beam spot with the present invention, a desiredcorneal ablation treatment can be effected without the laser beam shotsbecoming incident on undesired locations of target tissue orunderablating intended targets thereby enhancing the accuracy of theresculpting algorithm and procedure.

In the first embodiment of the present invention, as set out in FIGS. 1to 13, the laser beam spot is scanned along a path which passes over aknife-edge, (or any other such reference-edge), having a photodetectorpositioned therebehind. Preferably, the laser beam is orientedperpendicular to the plane of the reference-edge during the scanning. Invarious approaches, the laser beam can be scanned across thereference-edge and onto the photodetector, or across the photodetectorand onto the reference-edge.

By measuring the output of the photodetector, it is possible todetermine the intensity, size, shape and position of the laser beam spotduring the scanning, as follows.

FIG. 1 shows a perspective view of a laser beam 18 which is directeddownwardly from a laser source (not shown) towards a reference-edge 30and photodetector 40. Laser beam 18 is “scanned”, (ie: moved across,while remaining generally perpendicular to), a reference-edge 30 andphotodetector 40. An example of scanning is shown in FIG. 16 in whichlaser beam 18 is scanned across a measurement/alignment tool 100, whichmay comprise reference-edge 30 and photodetector 40. Specifically,galvanometer 120 is rotated to scan laser beam 18 across the surface ofalignment tool 100 from the position shown as beam 18A to the positionshown as beam 18B.

Returning to FIG. 1, laser beam 18 is thus scanned across reference-edge30 and photodetector 40 in direction D. Photodetector 40, (which maypreferably comprise a bulk photodetector), is positioned behindreference-edge 30 as shown. FIG. 2 shows a top plan view correspondingto FIG. 1 at the moment in time during the scanning where center 25 oflaser beam spot 20 is positioned exactly at the edge of reference-edge30. As can be seen, should laser beam spot 20 have a circular shape asillustrated, a first half 22 of laser beam spot 20 will be incident onphotodetector 40 at the moment in time during the scanning where center25 of laser beam spot 20 is positioned exactly over the edge ofreference-edge 30.

FIGS. 3A, 3B, and 3C show the sequential movement of laser beam spot 20as laser beam 18 is scanned across reference-edge 30 and ontophotodetector 40 during the scanning. FIG. 4 shows the correspondingintensity of output signal S from photodetector 40 taken over timeduring the scanning of beam spot 20 across reference-edge 30 and ontophotodetector 40. The intensity of output signal S of photodetector 40will correspond to the area of beam spot 20 which is not blocked byreference-edge 30 and is therefore directly incident on photodetector40. Specifically, the intensity of signal S can be represented for aGaussian pulse as follows:S = ∫₀^(x)(spot  intensity  profile  in  2D)𝕕x

-   -   or for a “top hat” pulse, (in which the energy distribution is        substantially uniform across the cross-section of the pulse), as        follows: $S = {\int_{0}^{x}\sqrt{x^{2} + {y^{2}{\mathbb{d}x}}}}$

Points P1, P2 and P3 on FIG. 4 illustrate the intensity of output signalS at the moments in time when beam spot 20 is positioned as shown inFIGS. 3A, 3B and 3C respectively. For a generally circular beam spot 20,the intensity of output signal S will be in the shape of an S-shapedcurve as shown in FIG. 4, as follows.

When beam spot 20 is positioned fully over reference-edge 30 as is shownin FIG. 3A, the photodetector will typically emit only a small signalintensity N, representing noise in the system. As beam spot 20 isscanned across reference-edge 30, progressively more of the area of thebeam spot 20 will reach photodetector 40, increasing the intensity ofthe photodetector's output signal S. When beam spot 20 reaches theposition illustrated in FIGS. 2 and 3B, such that center 25 of beam spot20 is positioned directly at reference-edge 30, first half 22 of beamspot 20 will be incident upon the photodetector 40. Accordingly, signalS will reach approximately ½ of its maximum signal intensity at pointP2. Finally, when beam spot 20 eventually reaches the positionillustrated in FIG. 3C, at which the entire beam spot 20 is incidentupon photodetector 40, signal S will reach its maximum signal intensityat point P3.

In a preferred aspect of the present invention, the intensity of laserbeam 18 is determined by measuring the maximum output signal of thephotodetector at point P3 when the laser beam spot is fully incident onthe photodetector and is not blocked by the reference-edge.

In another preferred aspect of the present invention, the area of laserbeam spot 20 is determined by taking the integral of the area undercurve S between points P1 and P3 since this area will correspond to thefull area of beam spot 20 which becomes incident upon photodetector 40from the beginning of the scanning as shown in FIG. 3A to the end of thescanning as shown in FIG. 3C.

In another preferred aspect of the invention, the location of center 25of laser beam spot 20 is determined. As explained above, center 25 oflaser beam spot 20 passes over reference-edge 30 when the intensity ofoutput signal S reaches point P2, being ½ of the intensity of outputsignal S at point P3. Due to the presence of a small noise signal N atpoint P1, it may be difficult to determine when the output signalintensity is at point P2. Accordingly, in a preferred approach, P2 isfound by determining a point midway between a first fraction of themaximum signal output and a second fraction of the maximum signaloutput, wherein the first and second fractions add together to themaximum signal output.

For example, a point P4 is located where the signal intensity equals 10%of the maximum signal output at point P3. Similarly, a point P5 islocated where the signal intensity equals 90% of the maximum signaloutput at point P3. After locating points P4 and P5 on the signal curve,point P2 is then located centrally therebetween. It is to be appreciatedthat points P4 and P5 could also be 30% and 70%, or 15% and 85%, or anyother combination of respective percentages which add together to 100%of the maximum signal intensity at point P3.

The speed of the scanning can be known either through position feedbacksystems or by determining the speed and time of the scanning. Knowingthe speed of the scanning, (which corresponds to the rate of rotation ofgalvanometer 120), and determining the moment in time at which P2 isreached, (ie: when the center 25 of beam spot 20 is positioned atreference-edge 30), the location of center 25 is thus determined.

In another preferred aspect of the present invention, the width of beamspot 20 in scanning direction D is determined as follows. Referringfirst to FIG. 3A, a leading edge 21 of beam spot 20 is positioned atreference-edge 30, (as represented by point P1 in FIG. 4). At thecommencement of scanning, leading edge 21 will start to become incidentupon photodetector 40, (as represented in FIG. 4 by the output signalintensity of the photodetector just beginning to increase). Referring tothe end of the scanning as shown in FIG. 3C, a trailing edge 23 willbecome incident upon photodetector 40 as shown, (as represented by pointP3 in FIG. 4 when the output signal intensity of the photodetector stopsincreasing).

Knowing the speed of movement of laser beam scanning in direction D,(either by knowing the speed and time during the scanning or through aposition feedback system), the moments in time when P1 and P3 arereached can be determined. As such, the width of laser beam spot 20,(which begins its passage over photodetector 40 at point P1 and endspassage at point P3), can easily be calculated.

In other preferred aspects of the invention, the shape of the laser beamspot 20 is determined by measuring the rate of change of output signal Sduring the scanning.

For example, FIG. 5 illustrates an oval shaped laser beam spot 20A beingscanned across reference-edge 30 and photodetector 40. Laser beam spot20A is elongated in direction D, as shown. The intensity of the outputsignal S corresponding to scanning laser beam spot 20A acrossreference-edge 30 and photodetector 40 is shown in FIG. 6. As can beseen, the rate of change of the output signal S of photodetector 40between points P1 and P3 is more gradual than was illustrated in FIG. 4,(shown by the greater amount of time separating points P1 and P3 in FIG.6 as compared to FIG. 4). The more gradual the rate of change of theoutput signal S in FIG. 6 thus indicates that laser beam spot 20A ismore elongated in direction D than circular-shaped laser beam spot 20.

Conversely, FIG. 7 illustrates an oval beam spot 20B, being scannedacross reference-edge 30 and photodetector 40. Laser beam spot 20B iselongated in a direction perpendicular to direction D, as shown. FIG. 8shows the intensity of output signal S corresponding to the scanning ofFIG. 7. As can be seen, the rate of change of output signal S is muchfaster than was shown in FIG. 4, (as shown by the smaller amount of timebetween points P1 and P3 in FIG. 8 as compared to FIG. 4). The fasterrate of change of the output signal S in FIG. 8 thus indicates thatlaser beam spot 20B is more elongated in a direction perpendicular todirection D than circular-shaped laser beam spot 20.

In another aspect of the present invention, the shape of laser beam spot20 can be determined by measuring the symmetry of output signal S duringthe scanning. As such, asymmetries and/or eccentricities of laser beamspot 20 are determined as follows. Referring to FIG. 9, an eccentricexaggerated “teardrop-shaped” laser beam spot 20C is scanned acrossreference-edge 30 and photodetector 40. Using the novel approaches setout above, the leading edge 21C will be located at point P1, the spotcenter 25C will be located at point P2 and the trailing edge will belocated at point P3 on FIG. 10. As can be seen, point P2 (at Conversely,FIG. 7 illustrates an oval beam spot 20B, being scanned acrossreference-edge 30 and photodetector 40. Laser beam spot 20B is elongatedin a direction perpendicular to direction D, as shown. FIG. 8 shows theintensity of output signal S corresponding to the scanning of FIG. 7. Ascan be seen, the rate of change of output signal S is much faster thanwas shown in FIG. 4, (as shown by the smaller amount of time betweenpoints P1 and P3 in FIG. 8 as compared to FIG. 4). The faster rate ofchange of the output signal S in FIG. 8 thus indicates that laser beamspot 20B is more elongated in a direction perpendicular to direction Dthan circular-shaped laser beam spot 20.

In another aspect of the present invention, the shape of laser beam spot20 can be determined by measuring the symmetry of output signal S duringthe scanning. As such, asymmetries and/or eccentricities of laser beamspot 20 are determined as follows. Referring to FIG. 9, an eccentricexaggerated “teardrop-shaped” laser beam spot 20C is scanned acrossreference-edge 30 and photodetector 40. Using the novel approaches setout above, the leading edge 21C will be located at point P1, the spotcenter 25C will be located at point P2 and the trailing edge will belocated at point P3 on FIG. 10. As can be seen, point P2 (at Conversely,FIG. 7 illustrates an oval beam spot 20B, being scanned acrossreference-edge 30 and photodetector 40. Laser beam spot 20B is elongatedin a direction perpendicular to direction D, as shown. FIG. 8 shows theintensity of output signal S corresponding to the scanning of FIG. 7. Ascan be seen, the rate of change of output signal S is much faster thanwas shown in FIG. 4, (as shown by the smaller amount of time betweenpoints P1 and P3 in FIG. 8 as compared to FIG. 4). The faster rate ofchange of the output signal S in FIG. 8 thus indicates that laser beamspot 20B is more elongated in a direction perpendicular to direction Dthan circular-shaped laser beam spot 20.

In another aspect of the present invention, the shape of laser beam spot20 can be determined by measuring the symmetry of output signal S duringthe scanning. As such, asymmetries and/or eccentricities of laser beamspot 20 are determined as follows. Referring to FIG. 9, an eccentricexaggerated “teardrop-shaped” laser beam spot 20C is scanned acrossreference-edge 30 and photodetector 40. Using the novel approaches setout above, the leading edge 21C will be located at point P1, the spotcenter 25C will be located at point P2 and the trailing edge will belocated at point P3 on FIG. 10. As can be seen, point P2 (at whichsignal intensity is ½ of that at P3), is not centered between points P1and P3, but rather is closer to P1, thus indicating that laser beam spot20C has a somewhat eccentric shape with its center 25C being closer toleading edge 21C than to trailing edge 23C.

As described above, the present invention provides systems for measuringthe intensity, size and shape profiles of a laser beam spot in thedirection in which it is scanned over a reference-edge and onto aphotodetector.

In additional preferred aspects of the present invention, the size,shape and position of the laser beam spot are determined in twodirections, as follows. Referring to FIG. 11, a beam spot 20 is moved ina first direction D1 across edge 31 followed by movement in a secondperpendicular direction D2 across edge 33. In this illustration, edges31 and 33 together form a corner to reference-edge 30.

Measuring the output signal of photodetector 40 as laser beam spot 20 isscanned across edge 31 using the above described techniques, thepositions of leading edge 21, trailing edge 23 and center 25 can bedetermined. Knowing the positions of leading edge 21 and trailing edge23, width W1 in direction D1 can be calculated. Subsequently, laser beamspot 20 is scanned in perpendicular direction D2 across edge 33. As aresult, the positions of side edges 27 and 29, and center 25 can bedetermined using the above described techniques. Knowing the positionsof side edges 27 and 29, width W2 in direction D2 can then becalculated.

FIG. 12 illustrates an arrangement similar to that of FIG. 11, butinstead using separate photodetectors 40A and 40B. FIG. 13 illustratesyet another arrangement, instead using two separate perpendicularreference-edges 32 and 34 and two separate photodetectors 40A and 40Bpositioned thereunder as shown.

After determining the size and shape of laser beam spot 20 uponphotodetector 40, the laser beam can then be safely directed at targettissue in the cornea of a patient's eye, knowing the exact size andshape of the beam spot which will be incident upon the target tissue.Preferably, the cornea can be sculpted to a desired shape by repeatedapplication of the laser beam to a number of different sites in apattern on the cornea. Using the present invention, the size and shapeof the laser beam spot can be precisely determined prior to, orconcurrently with, successive applications of the laser beam to thecornea.

For example, as shown in FIGS. 16 and 17, laser beam 18 can bealternatingly re-directed between a calibration tool 100 and thepatient's cornea 130. Calibration tool 100 may preferably comprisereference-edge 30 and photodetector 40 operating as described above.Referring to FIG. 16, laser beam 18 can be repeatedly reflected as beam18C by galvanometer 120 to a patient's cornea 130, (subsequently to thescanning of beam 18 across tool 100, from the position shown as beam 18Ato 18B). Referring to FIG. 17, tool 100 can instead be repeatedly movedback and forth to the position shown in phantom as tool 100A. As such,laser beam 18 is periodically interrupted in its application on cornea130 when tool 100 is positioned in the path of the laser beam todetermine the intensity and shape profiles of laser beam spot 20. Theprocess of repeatedly scanning beam 18 across alignment tool 100, orrepeatedly removing and replacing tool 100 in the beam path, (therebyrepeatedly determining the size and shape of laser beam spot 20), andthen repeatedly re-sculpting cornea 130 by laser ablation ensures thesize and shape of laser beam spot 20 do not change over time during theablation of the patient's cornea.

As illustrated in FIG. 18, a beam splitter 250 can also be used todirect a first portion 19A of beam 18 to tool 100 while simultaneouslydirecting a second portion 19B of beam 18 to cornea 130. Using thearrangement of FIG. 18, real time measurement of both intensity andshape profiles of beam spot 20 upon cornea 130 can be achieved while thetissues of the cornea are ablated.

Also shown in FIGS. 16, 17 and 18 are a computer 124 to record theintensity of the output signal of photodetector 40 over time, therebygenerating both intensity and shape profiles of laser beam spot 20.Additionally, computer 124 is adapted to calculate preferred patterns oflaser beam spot application on cornea 130 from the intensity and shapeprofiles of laser beam spot 20. As such, cornea 130 can sculpted to adesired shape. Additionally, a monitor 126 is adapted to display awaveform representing the intensity of the output signal ofphotodetector 40 over time.

In another preferred aspect, tool 100 can be used to align the targetingoptics of the laser delivery system. Specifically, after locating center25 of laser beam spot 20 as it is scanned across photodetector 40, thebeam delivery system (including galvanometer 120) can be preciselyaligned to compensate for any difference between the position of thelaser beam as determined by targeting optics 122, and that indicated bytool 100. A suitable material for tool 100 which fluoresces but does notablate is preferred. Such material may comprise a white stock paper or awhite business card. Also, a suitable fluorescent plate material whichcan be purchased from Startech Inc, of Connecticut can be used.

In a second embodiment, measurement/alignment tool 100 comprises ascreen 105 which fluoresces in response to laser light incident thereon,as illustrated in FIGS. 14 to 15B. Referring to FIG. 14, laser beam 18is directed incident to screen 105, causing screen 105 to fluoresce inthe region of beam spot 20. An operator 200 looking through targetingoptics 122, (which preferably comprises a system microscope), viewsfluorescing of beam spot 20, as shown in FIG. 15A. Targeting optics 122displays a reticle 110 to operator 200, and the operator adjusts thelaser beam delivery optics so that the fluorescing beam spot is alignedwith the reticle.

Advantageously, adjusting the location of beam spot 20 can be effectedusing the beam scanning mechanism. This may significantly facilitatealignment, as the system microscope need not be moved with a precise X-Yadjustment mechanism. Instead, the targeting signals transmitted to thegalvanometric laser beam delivery optics can be selectively altered oroffset to aim the beam tat the target location. Scanned accuracy may beenhanced by moving the beam between a plurality of target locations, andby individual beam shot targets using the signal offsets throughout theresculpting procedure. In alternative embodiments, the beam deliveryoptics may be mechanically adjusted to move beam spot 20 between thecross-hairs of reticle 110, thereby aligning the targeting optics of thelaser beam delivery system.

In some embodiments, tool 100 may be removably positioned at or near thelocation which will be occupied by the eye during refractiveresculpting. Tool 100 may be held by a swing-away arm or the like in aconventional manner. To set or check the system prior to a resculptingprocedure, the operator enters an alignment mode. In this mode, reticle110 remains stationary, and the laser fires to induce fluorescence atbeam spot 20. The beam spot may be moved by the operator via an inputdevice such as a joystick, mouse, switches, or the like which adjuststhe beam delivery optics by changing the signal sent to thegalvanometers. The laser beam would again fire producing a new laserspot 20, and the operator would continue to adjust the signal offsetsuntil the laser beam is coincident with the laser beam. When coincidenceis achieved, the operator can press a button (or provide any alternativesignal to the system) and the system computer will then store the offsetsignals for determining the ablation center. Typically, the reticle willalso be used to align the eye with the system after the tool is movedout of the way.

While the exemplary embodiments have been described in detail forclarity of understanding and by way of example, a variety of changes,adaptations, and modifications will be obvious for those of skill in theart. For example, a variety of scanning beam delivery systems might beused, including scanning systems which have a lens that may be variablyoffset from the beam axis or axes to image one or more laser beams at alaterally offset target location. The invention might be used with awide variety of ablation planning protocols or algorithms, and providesinput to such algorithms which can enhance their accuracy. Hence, thescope of the invention is limited solely by the appended claims.

1. A method of calibrating a scanning laser beam delivery system, themethod comprising: positioning a calibration tool at a target location;directing a laser beam onto the tool with the scanning beam deliverysystem; sensing the laser beam using the tool; adjusting the scanningbeam delivery system in response to the sensed laser beam; and directingthe laser beam toward a patient's cornea and sculpting the cornea withthe adjusted system.
 2. The method of claim 1, wherein the scanning beamdelivery system is a laser eye surgery system.
 3. The method of claim 1,wherein a galvanometric mirror is used for directing the laser beamtoward the patient's cornea.
 4. The method of claim 1, furthercomprising removing the tool prior to sculpting the cornea with theadjusted system.
 5. The method of claim 4, further comprising replacingthe tool at the target location after sculpting the cornea, andrepeating the directing, sensing and adjusting steps, wherein thedirecting step does not ablate the tool.
 6. The method of claim 1,wherein directing the laser beam onto the tool comprises scanning thelaser beam in a path across a reference-edge and onto a photodetector.7. The method of claim 6, wherein sensing the scanned laser beam usingthe tool comprises measuring an output signal from the photodetectorduring the scanning, the output signal corresponding to an area of thelaser beam incident on the photodetector during the scanning.
 8. Themethod of claim 7, wherein sensing the scanned laser beam using the toolcomprises determining dimensions of the laser beam by integrating anintensity of the photodetector signal output during the scanning.
 9. Themethod of claim 7, wherein sensing the scanned laser beam using the toolcomprises locating a center of the laser beam by determining when theoutput signal of the photodetector reaches half of a maximum outputsignal strength during the scanning.
 10. The method of claim 9, whereinlocating the center of the laser beam by determining when the outputsignal of the photodetector reaches a mid-point signal strength half-waybetween a first fraction of the maximum signal strength and a secondfraction of the maximum signal strength, wherein the first and secondfractions of the maximum signal strength add together to equal themaximum signal strength.
 11. The method of claim 7, wherein sensing thescanned laser beam using the tool comprises determining a width of thelaser beam in a direction of scanning by: locating a leading edge of thelaser beam by determining when the photodetector of the tool begins toemit an output signal indicative of the laser beam being incidentthereon; locating a trailing edge of the laser beam by determining whenthe output signal of the photodetector reaches a maximum output signal;and determining a spacing between the leading edge and the trailing edgeof the laser beam.
 12. The method of claim 1, wherein directing thelaser beam onto the tool comprises scanning the laser beam across atarget which fluoresces in response to the laser beam being incidentthereon.
 13. The method of claim 12, wherein sensing the scanned laserbeam using the tool comprises viewing the fluoresced target throughtargeting optics of the laser beam delivery system.
 14. The method ofclaim 13, wherein adjusting the system in response to the sensed laserbeam comprises aligning laser beam delivery optics of the laser beamdelivery system with the targeting optics until a viewed position of thefluoresced target is coincident with a target position of the targetingoptics.
 15. The method of claim 14, wherein the targeting optics of thelaser beam delivery system comprises a reticle.
 16. A method ofcalibrating a scanning laser beam delivery system, the methodcomprising: positioning a calibration tool at a target location;directing a laser beam onto the tool with the scanning beam deliverysystem; sensing the laser beam using the tool; adjusting the scanningbeam delivery system in response to the sensed laser beam; and removingthe tool and sculpting the cornea with the adjusted system.
 17. Themethod of claim 16, further comprising replacing the tool at the targetlocation after sculpting the cornea, and repeating the directing,sensing and adjusting steps, wherein the directing step does not ablatethe tool.
 18. A method of calibrating a scanning laser beam deliverysystem, the method comprising: positioning a calibration tool at atarget location; directing a laser beam onto the tool with the scanningbeam delivery system by scanning the laser beam in a path across areference-edge and onto a photodetector; sensing the laser beam usingthe tool; and adjusting the scanning beam delivery system in response tothe sensed laser beam.
 19. The method of claim 18, wherein sensing thescanned laser beam using the tool comprises measuring an output signalfrom the photodetector during the scanning, the output signalcorresponding to an area of the laser beam incident on the photodetectorduring the scanning.
 20. The method of claim 19, wherein sensing thescanned laser beam using the tool comprises determining dimensions ofthe laser beam by integrating an intensity of the photodetector signaloutput during the scanning.
 21. The method of claim 19, wherein sensingthe scanned laser beam using the tool comprises locating a center of thelaser beam by determining when the output signal of the photodetectorreaches half of a maximum output signal strength during the scanning.22. The method of claim 21, wherein locating the center of the laserbeam by determining when the output signal of the photodetector reachesa mid-point signal strength half-way between a first fraction of themaximum signal strength and a second fraction of the maximum signalstrength, wherein the first and second fractions of the maximum signalstrength add together to equal the maximum signal strength.
 23. Themethod of claim 19, wherein sensing the scanned laser beam using thetool comprises determining a width of the laser beam in a direction ofscanning by: locating a leading edge of the laser beam by determiningwhen the photodetector of the tool begins to emit an output signalindicative of the laser beam being incident thereon; locating a trailingedge of the laser beam by determining when the output signal of thephotodetector reaches a maximum output signal; and determining a spacingbetween the leading edge and the trailing edge of the laser beam.
 24. Amethod of calibrating a scanning laser beam delivery system, the methodcomprising: positioning a calibration tool at a target location;directing a laser beam onto the tool with the scanning beam deliverysystem by scanning the laser beam across a target which fluoresces inresponse to the laser beam being incident thereon; sensing the laserbeam using the tool by viewing the fluoresced target through targetingoptics of the laser beam delivery system; and adjusting the scanningbeam delivery system in response to the sensed laser beam by aligninglaser beam delivery optics of the laser beam delivery system with thetargeting optics until a viewed position of the fluoresced target iscoincident with a target position of the targeting optics.
 25. Themethod of claim 24, wherein the targeting optics of the laser beamdelivery system comprises a reticle.